1. Field of the Invention
This invention relates to intumescent vermiculite flakes which resist combustion and expand when subjected to high temperatures and are suitable for mounting a monolithic catalytic element in the casing of a catalytic converter. This invention also relates to a method of making the intumescent vermiculite flakes.
2. Description of the Related Art
Intumescent minerals such as flakes of vermiculite ore resist combustion and expand when subjected to high temperatures, and are used in catalytic converter mountings, caulks and putties for fire barriers, and in high-temperature gaskets. In a catalytic converter, a monolithic catalytic element (usually ceramic and herein referred to as a "monolith") can be mounted in a metallic casing by mounting material containing vermiculite flakes that expand when heated, thus compensating for the greater thermal expansion of a metallic casing as compared to a ceramic monolith. The mounting typically is in the form of a flexible mat which is wrapped around the monolith. The mat preferably incorporates inorganic fibrous material plus inorganic binder and/or organic binder which improves the integrity of the mat and thus makes it easier for the mat to be wound onto a monolith and to remain in place in the metallic casing.
Intumescent vermiculite mats are disclosed in co-assigned U.S. Pat. Nos. 3,916,057 (Hatch et al.) and U.K. Pat. No. 1,513,808 (issued Jun. 7, 1978). However, as is explained in co-assigned U.S. Pat. No. 4,305,992 (Langer et al.), the intumescent vermiculite has a region of negative expansion (contraction) between about 100.degree. and 400.degree. C. that can cause problems in each of the aforementioned uses. In a catalytic converter, this can cause the monolith to become loose from the casing during warm-up of the catalytic converter. To guard against such loosening, the initial expansion temperature of the intumescent vermiculite should be as low as possible, and the intumescent vermiculite should not experience any negative expansion.
The Langer patent subjects flakes of vermiculite ore to an ammonium cationic exchange, both to reduce the degree of negative expansion and to reduce the temperature at which the intumescent mats begin to expand. U S. Pat. No. 4,746,570 (Suzaki et al.) likewise subjects flakes of vermiculite ore to a cationic exchange plus a subsequent amine treatment in order to improve heat resistance and resiliency after repeated flexing at the high temperatures to which catalytic converters are subjected in use.
As explained in co-assigned U.S. Pat. No. 4,617,176 (Merry), pressure exerted against the monolith by intumescent vermiculite ore can be so high that, when combined with other factors, the monolith can experience "ring-off" cracks. In severe cases, the monolith is completely severed into two pieces. The Merry patent solves the ring-off problem by employing a flexible intumescent planar mat or sheet that is corrugated, i.e., it has a generally sinusoidal wave pattern along both its lengthwise edges, thus reducing forces exerted against the monolith. However, the corrugations of the intumescent sheet leave gaps that tend to allow heat to escape to the casing.
The Merry patent treats monolithic diesel particulate filters in the same manner as monolithic catalytic converters (herein, the term "catalytic converter" is meant to encompass monolithic diesel particulate filters). Like ordinary catalytic converters, many diesel monolithic particulate filters have a ceramic monolith mounted in a metallic casing, and the mounting can be a sheet of intumescent material.
In addition to the above-discussed problems, flakes of intumescent vermiculite ore experience greater expansion when first heated to high temperatures than in subsequent heating cycles. This thermal degradation results in gradually reduced holding forces at operating temperatures and, in mountings of catalytic converters, may also result in gradual erosion of the vermiculite by exhaust gases.